A significant contributor to the increase of net simple and combined cycle efficiencies for large industrial gas turbines is the ability to achieve tighter rotor/stator tip clearances. An important factor in the ability to control and maintain clearances is the ability to control the shape of the turbine casing. Circumferential thermal variations in the casing temperature can lead to shape distortions and thereby negatively impact efficiencies.
Large industrial gas turbines using advanced technologies to achieve higher simple cycle and combined cycle efficiencies are sensitive to local environment conditions. Ventilation of the enclosure or compartment in which the turbine is located is required in order to remove excess heat, and to minimize by dilution the buildup of hazardous gases in the enclosure. However, for large industrial gas turbines, several practical barriers arise: the amount of heat to be removed; the size of the compartment to be ventilated with its complexity of hardware and possible stagnate flow zones; and, the buoyancy dominated natural convection that may result in thermal gradients in casing surface temperature. The historical ventilation methodology results in a thermal profile of the turbine casings that adversely affects shape, and therefore clearances (or rubs), and thus also net simple and combined cycle efficiencies.
Two approaches have been used to achieve the requirements of heat removal and hazardous gas dilution. One is ventilation using a xe2x80x9cchimneyxe2x80x9d concept and the other is the use of insulation.
The chimney concept employs inlets placed low he turbine enclosure or compartment walls, outlets on the enclosure roof, and a blower fan also on the roof of the enclosure (or, if the enclosure is located within a larger building, then on the roof of the building). The enclosure and ducting is negatively pressurized to minimize the area which must be classified as xe2x80x9chazardousxe2x80x9d due to the presence of potentially hazardous gas leak sources. With the general flow pattern transverse to the axis of the turbine, and hence the turbine casing, the result is essentially a tube-in-cross flow arrangement where air comes in at the bottom of the enclosure, passes substantially vertically over the horizontally oriented turbine casing and exits the top of the enclosure. Along with a natural convection plume from the machine, a cold casing bottom and hot casing top is likely to result. This temperature profile results in a thermal distortion due to length along the top of the casing growing more than that of the bottom. On the occasion when dilution ventilation is also a concern, additional inlets are added to try and control the temperature gradients, but in many instances, the additional inlets resulted in impingement cooling of certain portions of the casing, also creating an adverse thermal effect.
The use of insulation is prevalent among many producers of large industrial gas turbines. Insulation has the advantage of minimizing the amount of heat rejected to the enclosure, and reducing the thermal sensitivity of the casing to local environment conditions. However, the large industrial gas turbines aimed at achieving higher efficiencies also utilize higher firing temperatures and higher pressure ratios. Both of these result in higher internal temperatures. Thus, special insulation materials must be used that are often more expensive, assuming they are even available. In some instances, the addition of insulation requires in casing flange bolts to be made of higher grade materials. Bolts made of this higher-grade material typically result in a cost increase that may be several times the additional cost of an axial ventilation system.
An xe2x80x9caxialxe2x80x9d ventilation system has been developed to provide better control of the ventilation air over the gas turbine casing within an enclosure to thereby minimize losses due to thermal variations. The xe2x80x9caxialxe2x80x9d flow concept has been developed to create a uniform flow along and around the casing, minimizing temperature variations. Because of the sweeping action inherent with the axial concept, dilution ventilation capabilities are also improved.
In order to create an axial flow over the turbine casing, an internal barrier wall is placed near the ventilation air inlet end of the enclosure. A plenum area or chamber is thus created between the enclosure wall and the internal barrier wall. It is in this area that inlet openings or louvers are located. From the plenum chamber, the ventilation air passes through an annular gap between the turbine casing and the barrier wall, causing the ventilation air to flow over the turbine in an axial direction. The pressure drop across the plenum inlet contributes to the uniformity of flow through the annular gap, while the increased velocity at the axial gap contributes to the effective length of impact of the axial flow. The momentum of this axial flow overcomes the otherwise dominant buoyancy effect, at least along that part of the turbine considered critical. Outlets are located in the roof, on the opposite end of the enclosure, from which the ventilation air is pulled by fans.
The arrangement of the turbine within the enclosure is such that it sits on two tall, concrete pedestals via bearings that are secured to opposite sides of the casing. Two horizontally directed inlets on opposite sides of the enclosure, downstream of the barrier wall, ventilate the area between the concrete pedestals, one inlet on each side of the enclosure. These inlets are slightly offset in the axial direction to induce mixing, and are located such that their flow does not impinge directly on the turbine casing. This lower volume of ventilating air eventually rises and is carried out of the enclosure with the larger volume of axially flowing air.
Other ventilation openings may be strategically located in the internal barrier wall so as to reduce or eliminate any stagnant or recirculation zones within the enclosure that would otherwise negatively impact dilution ventilation capability. With this arrangement, there is essentially a continuous sweep of the enclosure with ventilation air.
An additional feature of the ventilation system (which may have applicability in certain power plant layouts) pertains to personnel safety and relates to the manner in which persons may enter or exit the enclosure when the ventilation fans are running. Because of the amount of flow and the pressure losses of the flow through the various components, there is approximately 1 inch of water vacuum in the enclosure. With this level of vacuum, the force required to open a door may not be overcome by all individuals. Because of this, doors are provided through the internal barrier wall and through the enclosure aft of the barrier wall. While this feature is highly desirable for axial flow ventilation, it""s use does not have to be exclusive to such a concept. Another feature that has also been developed with this ventilation concept, but which is not exclusive to this arrangement, is the use of louvered doors.
In its broader aspects, therefore, the invention relates to a ventilation scheme for a turbine supported on pedestals within an enclosure having a roof, end walls and side walls with the turbine arranged parallel to the side walls, the ventilation scheme comprising ventilation air inlets located in a first end wall of the enclosure; a barrier wall located within the enclosure, proximate the first end wall to thereby create a plenum chamber, the barrier wall constructed to provide an annular gap between the barrier wall and a casing of the turbine to thereby direct ventilation air axially along the turbine; one or more ventilation air outlets located proximate a second, opposite end wall; and one or more fans for pulling ventilating air into the plenum chamber via the ventilation air inlets.
In another aspect, the invention relates to a combined cycle power plant comprising a gas turbine, a steam turbine and at least one generator, the gas turbine supported on axially spaced pedestals within the enclosure; ventilation air inlets located in a first end wall of the enclosure; a barrier wall located within the enclosure, proximate the first end wall to thereby create a plenum chamber, the barrier wall constructed to provide an annular gap between the barrier wall and a casing wall of the gas turbine to thereby direct ventilation air axially along the gas turbine; one or more ventilation air outlets located proximate a second, opposite end wall; and one or more fans for supplying ventilating air to the plenum chamber via the ventilation air inlets.
In still another aspect, the invention relates to a method of ventilating an enclosure having a roof, a pair of side walls and a pair of end walls, and wherein a gas turbine casing is supported on a plurality of pedestals in the enclosure such that a rotor of the turbine extends substantially parallel to said side walls, the method comprising:
a) establishing a plenum chamber adjacent one of the end walls;
b) pulling ventilation air into the plenum chamber;
c) causing the ventilation air to flow axially along at least as portion of the casing; and
d) removing the ventilation air proximate said other of said end walls.